[clang] [clang-format] Add support for absl nullability macros (PR #130346)

[Björn Schäpers via cfe-commits]

HazardyKnusperkeks wrote:

I still think Google should have to say what their style in clang-format is, extending to the default values of `AttributeMacros`.

I also wouldn't have such a big issue with adding it to the base style, because of the prefix and I think it's nice that Boost or Qt mostly work out of the box.

The only problem I see is how clang-format parses those lists from yaml. I think there is no possibility to choose between adding or overwriting, if you put `AttributeMacros` in your `.clang-format`, but it just overwrites the values.

https://github.com/llvm/llvm-project/pull/130346